Semiconductor leadframes are generally supplied leadframe sheets (or “strips”) stacked in a box. The leadframe sheets each comprise a plurality of leadframes, such as a sheet that is 10 leadframes wide by 20 leadframes long. The lid of the leadframe box typically includes a label that identifies the part number and lead finish associated with the leadframes. Between the respective leadframe sheets is commonly an interleaf provided for reinforcement to prevent tangling of the leadframe sheets. The interleaf can comprise a variety of materials including paper (e.g., lint-free paper), cardboard or a plastic film.
Assembly operations in a given assembly operation typically assemble a plurality of different die to a plurality of different leadframes. Leadframes of the same type (e.g., quad flat pack (QFP)) can have a variety of sizes (e.g., geometries) and arrangements, as well as a variety of different surface finishes.
Methods for ensuring that the proper leadframes are used in assembly include stamping or etching the part number on the side rails of the leadframe sheet. One problem with this method is the lead finish is added after the stamping or etching, and the lead finish later used may not be identifiable at the stamping or etching operation. Only part of the identification number (the mechanical design) is thus usually imprinted by stamping or etching the side rail of the leadframe sheet. Other problems associated with stamping or etching the side rail of the leadframe sheet include additional tooling being needed, and the possibility that the stamped or etched numbers can adversely affect the geometry (i.e., camber) of the leadframe sheets.
Ink printing after finishing the leadframes is another known method for leadframe identification. However, ink printing after finishing of the leadframes is not generally successful due to long drying times for inks on the metal leadframe surface. High speed printing (e.g., ink dot) inks do not stick well, or dry quickly enough on the metal leadframe surface to dry before stacking of the leadframe sheets to avoid smudging and resulting loss of discernable identifier information.
Accordingly, the part number is generally only printed on the lid of the leadframe box, which is separated from the stack of leadframes after the stack is loaded into the leadframe reservoir stack of the die mounter machine. A leadframe stack including leadframes from a plurality of leadframe boxes is generally placed in the reservoir stack of a die mounter at a given time for a given build operation.
Since identifiable part numbers (including its finish) are not printable on the leadframe sheet itself, mixing of leadframe part numbers, different finishes or leadframe sheets from different suppliers can be a problem. For example, during changeover at the die mounter, unused leadframe sheets in the mounter's reservoir stack are removed from the reservoir stack and returned to the leadframe box, which may no longer have its lid that has the part number identification information. Later use of these unused leadframe sheets can lead to mixing of leadframe part numbers, different finishes, or leadframe sheets from different suppliers.